1. Field of the Invention
The present invention relates to a development device provided in an electrophographic image forming apparatus (e.g., a copying machine, a laser beam printer, or a facsimile machine) The development device develops an electrostatic image formed on an image bearing member (e.g., photosensitive drum).
2. Description of the Related Art
In an image forming apparatus using electrophographic processes, a development device is configured to apply developer onto an electrostatic image formed on an image bearing member (e.g., photosensitive drum) to develop the electrostatic image as a toner image.
The development device includes a developing sleeve that carries and conveys developer. The developing sleeve is spaced from a photosensitive drum by a predetermined gap (hereinafter, referred to as “S-D (Sleeve-Drum) gap”). The development device, when it performs a development operation, applies a developing bias to the developing sleeve. The developing bias is a bias including a direct-current (DC) voltage component and an alternating-current (AC) voltage component. The developing sleeve includes regulating members (abutment rollers) provided at both ends thereof. The regulating members, when brought into contact with the photosensitive drum, maintain the above-described predetermined S-D gap between the developing sleeve and the photosensitive drum.
However, according to the above-described arrangement, if toner particles adhere to the surface of abutment rollers or the abutment rollers are worn down during a long-term use, the S-D gap may deviate from the originally set value.
FIG. 4 illustrates an example waveform of the developing bias. The waveform illustrated in FIG. 4 includes a first peak value Vmax applied during time T1 and a second peak value Vmin applied during time T2. An electric field formed when the first peak value Vmax is applied urges toner in a predetermined direction, from the developing sleeve to the photosensitive drum. An electric field formed when the second peak value Vmin is applied urges toner in the opposite direction, from the photosensitive drum to the developing sleeve. The bias having such a waveform is generally referred to as “duty bias.” An amplification circuit can generate a developing bias by amplifying a predetermined basic waveform. A waveform including a rectangular portion, which can realize a quick rise, is desired to develop a visible image having high image quality.
A transient response relative to the electrostatic capacitance of the S-D gap occurs when a developing bias is applied to the S-D gap. Therefore, a waveform resulting from an electrical potential change of the sleeve has a shape different from that of an input waveform.
FIG. 3 illustrates a general resistor-capacitor (RC) circuit and its transient characteristics. The RC circuit starts charging a capacitor upon closing a switch S. When a constant voltage is applied, the voltage across the capacitor changes with a rise time determined by a resistance R and a capacitance C. If a power source voltage V0 is high, the charging speed is high and the voltage across the capacitor rises quickly. The electrostatic capacitance in an image forming apparatus is determined according to the distance of the S-D gap, materials contained in the developer, and the shape of magnetic brush. For example, if the S-D gap is narrow, the electrostatic capacitance is large and a long time is required to store electric charge. Therefore, the waveform at a rise portion becomes dull (as indicated by a dotted line in FIG. 4). The developing performance deteriorates and the image density decreases.
Deterioration in developing performance is described below in more detail. FIG. 7 illustrates toner flying from a magnetic brush generated when a developing bias is applied to a developing sleeve 21. The electric field concentrates at a front edge of the magnetic brush where the gap between the magnetic brush and the photosensitive drum 11 is shortest. A repulsive force constantly acts on toner particles because of their electric charges having the same polarity. Therefore, the flying of toner particles successively occurs at or near the front edge of the magnetic brush. If the developing bias is dull in its rise portion, the electric field has a dull rise at the front edge of the magnetic brush. Therefore, the flying of toner particles starts at delayed timing. An AC bias alternately applies positive and negative electric fields to the S-D gap within a limited time. Accordingly, the amount of flying toner particles decreases when the flying of toner particles starts at delayed timing. As a result, the developing performance deteriorates or the image density decreases.
On the other hand, if the S-D gap is wide, the electrostatic capacitance is small and the time required for charging is short. However, the rising voltage may momentarily exceed the predetermined peak value Vmax (which is referred to as “overshoot”) as indicated in FIG. 5. In FIG. 5, a solid line represents a basic waveform and a dotted line represents a waveform causing an overshoot when the electrostatic capacitance is small. If the overshoot phenomenon repetitively occurs, the amount of conductive substances increases in the developer and abnormal discharge occurs in the S-D gap. If abnormal discharge occurs on or near the drum surface, a ring-shaped defective image (ring mark) appears on the drum surface at a corresponding position due to a decrease in the electrical potential.
Accordingly, applying a developing bias having a desired waveform is important to assure stable developing performance. To this end, performing initial setting for a sharp rise can maintain the shape of a waveform as indicated by the solid line in FIG. 5. For example, as discussed in Japanese Patent Application Laid-Open No. 2000-214665, a waveform shaping resistance can be serially connected to a developing device to adjust the deformation of a waveform if the deformation occurs due to initial differences of the individual developing device.
However, a change of the electrostatic capacitance C of the S-D gap is greatly dependent on the elapsed time because the electrostatic capacitance C is variable according to the distance of the S-D gap, materials contained in the developer, and the shape of magnetic brush. Therefore, even after each of the power source side and the developing device side is adjusted to assure a predetermined value as described in Japanese Patent Application Laid-Open No. 2000-214665, the waveform may deform due to a change in the electrostatic capacitance during a long-term use of the apparatus. The deformation in waveform greatly influences the quality of an image.
To solve the above-described problem, a method discussed in Japanese Patent Application Laid-Open No. 9-54487 includes applying a bias to a developing sleeve and changing a DC offset bias applied to the developing sleeve, or a peak-to-peak voltage of an applied rectangular voltage, according to the output current. The method prevents the toner density from varying when the gap between an image bearing member and a developer bearing member changes.
However, according to the method discussed in Japanese Patent Application Laid-Open No. 9-54487, it is required to change the DC offset voltage applied to the developing sleeve or the peak-to-peak voltage of the AC voltage. For example, if the DC offset voltage is changed, a Vback representing a difference between the developing electrical potential and an electrical potential of a dark portion decreases and carrier adhesion easily occurs. On the other hand, if the peak-to-peak voltage is changed, abnormal discharge easily occurs because of the increased peak-to-peak voltage.
A method discussed in Japanese Patent Application Laid-Open No. 2004-54297 includes calculating the resistance of a developing roller and the SD gap and changing the amplitude and the duty ratio of an applied AC voltage. The method can prevent the toner density from varying when the gap between an image bearing member and a developer bearing member changes.
However, similar to the method discussed in Japanese Patent Application Laid-Open No. 9-54487, the method discussed in Japanese Patent Application Laid-Open No. 2004-54297 cannot reduce deterioration in image quality (carrier adhesion or abnormal discharge) occurring due to changes in the amplitude and the duty ratio of the AC voltage.